What if you modify other parameters?
Open the .lks file by notepad. Check if COM1 is changed to COM4.
The code for modbus object looks like
Modbus1 = new Modbus (1, "COM4", 9600, "none", 8, 1, , 2000, 2000, 125, 125, 800, 100, 8, 4, 4, 500, true, false, , , 0, , false);
Ryan Shi
National Instruments